1. Field of the Invention
The present invention relates to a very fast analog-to-digital (A/D) converter and, more particularly, to a very fast A/D converter which first converts the input analog waveform into a phase-modulated signal and then uses broadband circuitry to achieve fast word-at-a-time digital encoding of the phase-modulated signal.
2. Description of the Prior Art
Analog-to-digital (A/D) converters are well known in the art, and are used to convert an analog input signal, which may be a voltage or a current signal, into an equivalent digital output signal. In this regard see, for example, U.S. Pat. Nos. 3,611,350 issued to L. M. Leibowitz on Oct. 5, 1971 and 3,701,144 issued to D. E. Fineran et al on Oct. 24, 1972. Although such A/D converters are at times designated high speed, they are limited in speed of operation to well below a 10 MHz sampling rate generally as a result of delays in the encoder logic used. Additionally, such prior art A/D converters are generally relatively complex and usually require a large number of elements.
To simplify the A/D converter circuitry and to permit an increase in the speed of operation, analog-to-Gray code converters have been devised. In this regard, see, for example, U.S. Pat. Nos. 3,587,089 issued to N. P. Elliott on June 22, 1971 and 3,829,853 issued to D. D. Freedman on Aug. 13, 1974. These arrangements have increased the speed of operation up to near the 10 MHz sampling rate. Although A/D converters having sampling rates which are much greater than 10 MHz have been devised, such A/D converters are extremely complex and are limited to a very few bits.
The problem remaining in the prior art, however, is to provide a relatively uncomplicated and economical A/D converter capable of converting an analog signal into a corresponding digital output signal at, for example, a sampling rate above 100 MHz.